Field of the Invention
The invention concerns a method for operating a magnetic resonance tomography apparatus, in particular the scanner thereof, wherein a scanning volume is subdivided in a slice direction into a number of scanning slices, wherein the scan data of each of the scanning slices are acquired by a scan sequence allocated to the scanning slice, each scan sequence includes at least one preparation pulse allocated to that scanning slice, by which an excitation takes place in the entire scanning volume. The invention also concerns a magnetic resonance tomography apparatus, and a non-transitory, computer-readable data storage medium encoded with programming instructions, for implementing such a method.
Description of the Prior Art
In the context of magnetic resonance tomography, imaging is often desired wherein an image contrast is optimized for particular molecular species. For example, it is usual in the context of medical imaging to reduce the influence of fat tissue on the imaging by various imaging techniques. For this purpose, use is made of the fact that water and fat molecules, in particular CH— and CHO— groups, have slightly different resonance frequencies. The chemical shift is in the region of 3.5 ppm. If, for example, a basic magnetic field of 1.5 T is used, then this corresponds to a frequency shift of 220 Hz.
In order to reduce the image contrast contribution of fat molecules, the short relaxation time of fat molecules can be utilized. For example, with the STIR technique, the fat molecules are excited with a 180° pulse and the scan takes place delayed such that the fat signal is reduced to approximately zero. A disadvantage of this type of scan is that signals from tissue portions that are to be scanned (i.e. for which diagnostic data are desired) are also attenuated.
Other approaches are based on so-called fat saturation, wherein the fat molecules are excited by a radio frequency pulse, for example a 90° pulse, and the existing transverse magnetization is completely dephased by a spoiler gradient. An example of this is the so-called CHESS technique. The approaches described are also combinable, as occurs, for example, in the SPAIR technique. In order to achieve chemical selectivity, an excitation takes place by radiation of a chemically selective preparation pulse in the whole scanning volume. However, such a chemically selective excitation is very sensitive to field inhomogeneities of the basic magnetic field since these detune the resonance frequency of the different molecular species locally.
It is known to determine inhomogeneities in the basic magnetic field in a magnetic resonance scanner so that such inhomogeneities can be compensated locally through a suitable control of field correction coils, also known as shim coils and/or an adaptation of pulse frequencies of an excitation pulse. If, however, larger scanning volumes are to be investigated, it is often not possible to balance inhomogeneities in the entire scan region, so that in the saturation techniques described, no optimum chemical selectivity is achieved. This results in the achievable image quality being lessened.